There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as cellular communication, satellites, portable computers, and electric vehicles to name but a few. Accordingly, there has been recent concerted efforts to develop high energy, cost effective batteries having improved performance characteristics.
Rechargeable or secondary cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being recharged by the application of an electrical charge thereto. Numerous advanced electrode systems have been developed for storing electrical charge. Concurrently much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities and performance of electrochemical cells.
Heretofore, electrolytes have been either liquid electrolytes as are found in conventional wet cell batteries, or solid films as are available in newer, more advanced battery systems. Each of these systems have inherent limitations and related deficiencies which make them unsuitable for various applications. Liquid electrolytes, while demonstrating acceptable ionic conductivities tend to leak out of the cells into which they are sealed. While better manufacturing techniques have lessened the occurrence of leakage, cells still do leak potentially dangerous liquid electrolytes from time to time. Moreover, any leakage lessens the amount of electrolyte available in the cell, thus reducing the effectiveness of the device.
Solid electrolytes are free from problems of leakage, however, they have traditionally offered inferior properties as compared to liquid electrolytes. This is due to the fact that ionic conductivities for solid electrolytes are often one to two orders of magnitudes poorer than a liquid electrolyte. Good ionic conductivity is necessary to insure a battery system capable of delivering usable amounts of power for a given application. Most solid electrolytes have not heretofore been adequate for many high performance battery systems.
One class of solid electrolytes, specifically gel electrolytes, have shown great promise for high performance battery systems. Gel electrolytes contain a significant fraction of solvents and/or plasticizers in addition to the salt and polymer of the electrolyte itself. One processing route that can be used to assemble a battery with a gel electrolyte is to leave the electrolyte salt and solvent out of the polymer gel system until after the cell is completely fabricated. Thereafter, the solvent and the electrolyte salt may be introduced into the polymer system in order to swell and activate the battery. While this approach (which is described in, for example, U.S. Pat. No. 5,456,000 issued Oct. 10, 1995) has the advantage of allowing the cell to be fabricated in a non-dry environment (the electrolyte salt in a lithium cell is typically hygroscopic) it offers problems with respect to performance and assembly. The gel electrolyte may lack sufficient mechanical integrity to prevent shorting between the electrodes while they are being bonded or laminated together with the electrolyte. The electrolyte layer thickness is typically on the order of 75 .mu.m, presumably to overcome this shorting problem and to help facilitate handling of the electrolyte material. When compared to the 25 .mu.m thickness typical for separators used in liquid lithium ion cells, this results in a significant reduction in the volumetric energy density for the cell.
Accordingly, there exists a need for anew electrolyte system which combines the properties of good mechanical integrity, as well as the ability to absorb sufficient amounts of electrolyte active species so as to produce an electrolyte with the high ionic conductivity characteristics of liquid electrolytes. The electrolytes so formed should also avoid excessive swelling and all of the problems associated therewith.